Teun Remmers

26 | Chapter 2 Discussion This study showed that increments of MVPA across a period of approximately 2.5 years were related to a significant decrease in BMI z-scores in heavier children. In heavier boys, increments in light PA were also related to a significant decrease in BMI z-scores, and in normal weight boys, somewhat similar decreases were found. Our finding concerning the association between MVPA and BMI in heavier children is supported by previous studies (17, 22), although more recent evidence is less clear for this association (40). As high BMI z-scores and low MVPA percentages at baseline are likely to subsequently regress to a mean value because of natural causes, this finding may also be influenced by the regression to the mean principle. However, the differential effects of PA in initially leaner, normal weight and heavier subgroups of children, between boys and girls, and within different intensities of PA, strengthen the impression that these results are not an artificial relationship caused by regression to the mean. In this study, the intensity of PA was based on accelerometry-based counts. One should bear in mind that equal levels of MVPA do not necessarily mean equal energy expenditure. A disadvantage of accelerometers is their inability to detect arm movements and external work (8). Thereby they underestimate the energy cost of certain activities, such as rowing, cycling, weight lifting or using the stairs. In addition, the energy expenditure depends on the child’s motor ability or physical fitness. Specifically, a physically fit child with high motor abilities may expend less energy during PA of equal intensity, compared to their relatively unfit counterparts. The advantage of using PA intensity levels instead of energy expenditure is that PA is not dependent upon these factors. Future studies should use objectively measured PA, accompanied by measures of activities that accelerometers are unable to detect. The differential effect of PA on BMI development between boys and girls is supported by one study (2). Although it is nowadays well understood that most boys spend more time being physically active than girls (38), it is unknown why girls seem to be less sensitive to changes in PA. Future studies are therefore encouraged to investigate this more thoroughly. In contrast to MVPA, light PA showed somewhat weaker associations with BMI development. This discrepancy is supported by several studies (10, 16, 28) which generally reported that PA of the highest intensity (i.e. vigorous PA) was more strongly related to decreases in adiposity compared to less intensive PA. One study showed that the effect of MVPA was independent of the time spent sedentary (33), which may indicate that vigorous PA may provide more benefit than just increasing energy expenditure.